The invention relates generally to location determination in wireless communications and, more particularly, in narrow bandwidth wireless communication systems.
Present telecommunication system technology includes a wide variety of wireless networking systems associated with both voice and data communications. An overview of several of these wireless networking systems is presented by Amitava Dutta-Roy, Communications Networks for Homes, IEEE Spectrum, pg. 26, December 1999. Therein, Dutta-Roy discusses several communication protocols in the 2.4 GHz band, including IEEE 802.11 direct-sequence spread spectrum (DSSS) and frequency-hopping (FHSS) protocols. A disadvantage of these protocols is the high overhead associated with their implementation. A less complex wireless protocol known as Shared Wireless Access Protocol (SWAP) also operates in the 2.4 GHz band. This protocol has been developed by the HomeRF Working Group and is supported by North American communications companies. The SWAP protocol uses frequency-hopping spread spectrum technology to produce a data rate of 1 Mb/sec. Another less complex protocol is named Bluetooth after a 10th century Scandinavian king who united several Danish kingdoms. This protocol also operates in the license-free 2.4 GHz band and advantageously offers short-range wireless communication between Bluetooth devices without the need for a central network.
The Bluetooth system provides a 1 Mb/sec data rate with low energy consumption for battery powered devices operating in the 2.4-GHz ISM (industrial, scientific, medical) band. The current Bluetooth system provides a 10-meter range and a maximum asymmetric data transfer rate of 723 kb/sec. The system supports a maximum of three voice channels for synchronous, CVSD-encoded transmission at 64 kb/sec. The Bluetooth system treats all radios as peer units except for a unique 48-bit address. At the start of any connection, the initiating unit is a temporary master. This temporary assignment, however, may change after initial communications are established. Each master may have active connections of up to seven slaves. Such a connection between a master and one or more slaves forms a xe2x80x9cpiconet.xe2x80x9d Link management allows communication between piconets, thereby forming xe2x80x9cscatternets.xe2x80x9d Any Bluetooth device can assume the role of master or slave. For example, typical Bluetooth master devices include cordless phone base stations, local area network (LAN) access points, laptop computers, or bridges to other networks. Bluetooth slave devices may include cordless handsets, cell phones, headsets, personal digital assistants, digital cameras, or computer peripherals such as printers, scanners, fax machines and other devices.
The Bluetooth protocol uses time-division duplex (TDD) to support bi-directional communication. Frequency hopping permits operation in noisy environments and permits multiple piconets to exist in close proximity. The frequency hopping scheme permits up to 1600 hops per second over 79 1-MHZ channels or the entire 2.4-GHz ISM spectrum. Various error correcting schemes permit data packet protection by ⅓ and ⅔ rate forward error correction. Further, Bluetooth uses retransmission of packets for guaranteed reliability. These schemes help correct data errors, but at the expense of throughput.
The Bluetooth protocol is specified in detail in Specification of the Bluetooth System, Version 1.0A, Jul. 26, 1999, which is incorporated herein by reference.
Techniques have been developed for identifying the geographic location of a wireless communication device, for example, in emergency situations or to provide travel directions. However, these techniques can be particularly difficult to implement when the devices are operating indoors. Global Positioning System (GPS) satellite reception may be impossible, and wireless telephony may be difficult at best in many locations, such as the inside of factories, high-rise buildings, parking garages, shopping malls, subway/train stations and airport terminals.
It is therefore desirable to provide the capability of identifying the geographic location of a wireless mobile communication device that is operating indoors.
Many conventional approaches to precision location identification make use of so-called xe2x80x9ctime of arrivalxe2x80x9d techniques. One difficulty with time of arrival techniques is the uncertainty of time, which can occur at several locations. For example, if it is desired to locate a particular wireless mobile communication device, and that device broadcasts a beacon in several time slots, with each time slot dedicated to a respective base station, then the uncertainty of the wireless mobile communication device""s clock can be a source of error in the location identification operation. If the base stations are operated with respectively independent clocks, then the uncertainty associated with the independent clocks can also be a source of error in the location identification operation.
It is therefore desirable to provide location identification techniques that avoid disadvantageous time uncertainties.
Some conventional techniques utilize a wide frequency signal for location identification. Such a wide bandwidth signal permits a very narrow pulse width, the timing of which can be precisely measured. However, this wide bandwidth signal is not available in narrow bandwidth wireless communication systems such as Bluetooth systems. For example, the communication bandwidth in Bluetooth systems is only 1 MHz. Therefore, the smallest bit length is 1 microsecond. Disadvantageously, an error of 1 microsecond in timing corresponds to a distance uncertainty of 300 meters (3xc3x97108 meters/secondxc3x9710xe2x88x926 seconds). Another problem encountered in systems such as Bluetooth is that the preamble defined by the Bluetooth protocol is only long enough to insure that all data bits are sampled without error. There is no determination of the start of a bit, or definition of the bit edge.
It is therefore desirable to improve the precision of location identification in narrow bandwidth wireless communication systems.
It is also desirable in view of the foregoing to provide location identification techniques that do not require the capability of determining a start bit or defining a bit edge.
The present invention determines, at several known locations, the phase difference between a known stable reference signal and a known signal output by the wireless mobile communication device that is being located. The location of the wireless communication device can be determined from the phase difference information obtained at the predetermined locations. This advantageously permits precise location estimation indoors, using a relatively narrow bandwidth signal, and also advantageously avoids the aforementioned problems of time uncertainties and bit definition. Further according to the invention, the approximate location of a wireless mobile communication device can be estimated by transmitting a message from the device at a predetermined power level, and determining where among a plurality of predetermined locations the transmitted message has been received.